6.4 Non-detachable connections
Non-detachable connections in vacuum technology are achieved by welding, brazing or fusing, or by metalizing or sintering with subsequent brazing. In recent years, vacuum-resistant adhesives have also come into use to join components for applications that do not involve UHV technology. The selected connection technology must be appropriately designed for the major requirements with respect to mechanical strength, temperature and alternating thermal loads, as well as the required gas-tightness. Material pairings such as metal-to-metal, glass-to-glass, glass-to-metal, metal-to-ceramic and glass-to-ceramic are being used more or less frequently in vacuum technology. Metals are most often joined by means of welding and brazing. In glass equipment, the individual glass components are joined through fusion. Non-detachable joints between metal and glass that are produced by fusing or metalizing and fusing are less frequent, and joints between metal and ceramic, which are produced by metalizing or sintering, are also less frequent.
In vacuum equipment, components of plain and stainless steel are usually welded together for vessels and joints. In addition, it is also possible to weld aluminum components together. To ensure that the welds that are produced are vacuum-tight, it is necessary to use proper materials that are free of cracks and voids, and whose surfaces are smooth and free of grease.
In addition, a special geometric design is also required that sometimes differs from the normal welded connections that are employed for non-vacuum applications. Wherever possible in terms of engineering, interior welds must be provided in order to avoid vacuum-side gaps and cracking, so-called latent leaks. If this is not possible, the weld must extend through to the vacuum side. Where necessary, a supplemental atmosphere-side weld can be employed to increase mechanical stability.
In this connection, it is important that this supplemental weld not be continuous in order to allow leak detection, if necessary, and have no air inclusions. In addition to the TIG welding process, microplasma welding also plays a role in vacuum technology, particularly for welding extremely thin-walled components and, to an increasing extent, in electron beam welding, which must be performed under vacuum.
In addition to welding, the brazing process is also used to join metals. Brazed joints at soldering temperatures of above 600 °C are used almost exclusively in vacuum technology. In order to eliminate the need for highly corrosive flux when soldering, which usually involves high vapor pressure, and in order to obtain oxide-free, high-strength joints, the soldering process is performed under vacuum or in a clean inert gas atmosphere. Soft solder joints are not suitable for vacuum applications. They typically cannot be baked out, have less mechanical strength and in addition to tin frequently contain other alloy components with high vapor pressures.
The fusing process is an alternative that is primarily used for joining glass components (in glass equipment) and for glass-to-metal connections. Glass-to-metal fusings are especially important in the production of vacuum-tight current feedthroughs, for bakable sight glasses and in the production of vacuum gauges. To fuse glass-to-metal transitions, the materials must be selected in such a manner that the thermal expansion coefficients of these materials are as similar to one another as possible throughout a broad temperature range. Numerous special alloys have been developed for this purpose that are known under trade names such as Fernico, Kovar, Vacon, Nilo, etc. Fusings with quartz glass are difficult to perform, as this material has an extremely low thermal expansion coefficient; no metal or metal alloy even comes close.
Ceramic-to-metal connections are used for highly bakable and highly insulating current feedthroughs. They are also employed for manufacturing high-performance transmitting tubes and for configuring ceramic vacuum chambers for particle accelerators at major physics research facilities. In the case of this connection technology, the ceramic, e.g. aluminum oxide (92 % to 98 % Al2O3), is pre-metalized at those points to be joined with the metal. In this connection, it is particularly important to ensure that the thin metal layer (molybdenum or titanium) crease an intensive connection with the ceramic substrate that is free of voids and pores. Applied to this is a layer of nickel; this enables a metal cap to be brazed on, for example, to which the current conductors of the current feedthrough are subsequently soldered.
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